High-spectral efficiency (bps/Hz) offered by Multiple-Input-Multiple-Output (MIMO) is present in many upcoming standards such as Long Term Evolution (LTE), LTE-Advanced, Wireless Local Area Network, 802.11n (WLAN), and Worldwide Interoperability for Microwave Access (WiMAX), 802.16e, for implementing high data rate modes such as 100 Mbps-1 Gbps. In MIMO communication, the original data stream to be sent is broken up into multiple streams and transmitted from different antennas at the same time in the same frequency band, resulting in the mentioned high spectral efficiency. A key issue with MIMO technology is the intentional interference caused by the transmission of independent data streams at the same time in the same frequency band. A receiver using MIMO technology thus has the additional burden of separating the streams from each other to achieve this spectral-efficiency. Hence, a key function of a MIMO receiver affecting the reception performance significantly is an equalizer.
The equalizer mitigates the effects of inter-symbol interference caused by the MIMO channel, i.e. performs detection, whereas a channel decoder, such as a turbo decoder, addresses the problem of recovering data, thus actual transmitted bits, from the equalized symbols. For complexity reasons, conventional receivers traditionally address equalization and decoding separately. These conventional receivers are still inferior when it comes to error rate performance compared to equalization schemes performing equalization and channel decoding jointly. Schemes performing equalization and channel decoding jointly are known as turbo equalization. For a turbo equalizer, Single-Input-Single-Output (SISO) or MIMO based receiver, soft input/soft output MIMO detector/equalizer, scrambler/de-scrambler interleaver/de-interleaver, and a turbo decoder providing soft output need to be used in order to iteratively improve Block Error Rates (BLER). For every turbo-equalizer iteration, the performance of the receiver is improved, i.e. the BLER drops until at a point where no further improvements are possible.
For each turbo-equalizer iteration a received signal is first equalized in the detector with a-priori inputs coming from the turbo decoder, thus from a previous iteration, or zero if the current iteration is the first iteration. Second the received signal is decoded in the turbo decoder itself whilst producing new soft-outputs which may be used as a-priori input in the next iteration.
Chun-Hao Liao et al., “Combining orthogonalized partial metrics: Efficient enumeration for soft-input sphere decoder”, IEEE 20th international symposium on personal, indoor and mobile radio communications (PIMRC 2009), IEEE, Piscataway, N.J., USA, 13 Sep. 2009, pages 1287-1291, XP031659905, ISBN 978-1-4244-5122-7, describes an algorithm in a sphere decoder and a modified pruning metric that is suitable for an iterative MIMO detector.